1. Technical Field
The embodiments herein generally relate to spur management techniques, and, more particularly, to digitally controlled spur management techniques for DC-DC converters integrated with an electrical circuit.
2. Description of the Related Art
DC-DC converters are switching regulators that provide well regulated voltage from unregulated input voltage (usually a battery in mobile applications). The DC-DC converter is very noisy due to the fact that it switches large current values in order to supply the total current of all digital and analog circuits supplied by its output. The switching frequency of the DC-DC converter may be load independent, in a continuous mode of operation, or load dependent in a discontinuous mode of operation. In a continuous mode of operation, spurs of the DC-DC switching frequency ‘fs’ (e.g., a sampling frequency) occur at only integer multiples of the switching frequency. Sampling frequency or a sample rate defines the number of samples per unit of time (usually seconds) taken from a continuous signal to make a discrete signal.
The ever shrinking size of MOS transistors enables the integration of more functionality in a smaller die area. The drive to lower the cost of consumer products has motivated the integration of analog circuits together with noisy digital circuits. Noise shielding techniques are used to minimize the performance degradation of noisy digital circuits that become in close proximity with the sensitive analog circuits. There remain, however, some noisy circuits, such as switching DC-DC converters that typically cannot be integrated with sensitive analog circuit due to their high level of switching noise. The spurs (e.g., harmonic distortion component) level of this switching noise at the frequency band of interest in the analog circuits is often unacceptable or at least uncontrollable.
Several coupling mechanisms (e.g. substrate, magnetic, power supply, etc.) cause these spurs to reach the sensitive analog circuits. This spur issue has prevented integration of DC-DC converters with spur-sensitive analog circuits and kept the DC-DC converter as an off-chip component. However, by making the DC-DC converter an off-chip component leads to increase in size and cost of the overall solution. Further, adding off-chip capacitors to filter the noise will not help if the DC-DC converter is integrated. This is because the high switching current has to flow from the on-chip power switches to the off-chip capacitor. Therefore, the size of the current loop becomes large, increasing magnetic coupling to sensitive on-chip analog RF circuits.
DC-DC switching noise coupling into the analog/RF section can degrade the performance of the entire receiver. In the case of a weak received desired signal level (limiting case would be a signal at near the sensitivity of the receiver), the digital noise level coupling into the input of the LNA may be on the same order of magnitude, indeed may even be larger, than the RF desired signal itself. In such cases, the total receiver sensitivity is degraded by an amount equal to the increase in receiver noise figure due to digital noise coupling.
Switching noise coupling into sensitive RF circuitry is the main bottleneck behind integrating an RF receiver on the same substrate, or even in the same package, with a switching DC-DC converter. One major form of coupling is magnetic (inductive). Unlike resistive and capacitive coupling, inductive coupling does not depend on material type or material depth. It is understood that any conductive closed loop with time varying electric current flowing through the loop will produce a magnetic field. Similarly, any conductive closed loop subjected to a time varying magnetic field will conduct electric current.
The simplest and most effective method to reduce the magnetic coupling effect is to increase the physical separation between the two loops. Due to the advancement in technology, the die size includes smaller feature sizes and hence the physical separation one can achieve between the digital (aggressor) and analog circuitry (victim) is constantly reducing. Hence, in modern nanometer technology, separating the DC-DC converter and analog circuits by large distances is no longer possible especially if a small die area is the target for lower cost and higher yield.
Scaling of integrated circuit technology has allowed the industry to integrated more functions on the same chip. One of the recent trends is to try to integrate the entire system into a single package (system-in-a-package or SiP) or a single chip (system-on-a-chip or SoC). If the DC-DC converter is to be integrated within the same package or in the same chip, it will be in very close proximity to the sensitive analog circuitry. If one or more of the spurs/harmonics of the DC-DC switching frequency falls in a frequency band of interest of the wireless receiver, it will generally corrupt the reception of the desired signal. Accordingly, there remains a need for a new technique to digitally control spurs for integrated DC-DC converters.